1. Field of the Invention
The present invention generally relates to a laser device, and a controller and method for controlling the laser device. More particularly, the present invention relates to a laser device supplied with a modulating signal on which a low-frequency component is superimposed, and a controller and method for controlling such a laser device.
2. Description of the Related Art
In a laser device equipped with no cooling means, a variation in the performance caused by temperature change is handled as a variation in quantum efficiency (ΔP/ΔI: P denotes optical power and I denotes drive current), particularly, differential quantum efficiency η. Thus, in order to stabilize the laser performance, a laser output is converted into current for monitoring the performance. The amplitude of the driver output for driving the laser device is controlled so that the average of the monitor current is kept constant.
Japanese Laid-Open Patent Application No. 3-278586 discloses an optical amplitude modulation apparatus having a mechanism for controlling the driver output amplitude to a fixed level. This apparatus is referred to as first conventional apparatus. As is shown in FIG. 1, the first conventional apparatus is made up of a laser diode (LD) 101, a photodiode (PD) 102, a low-pass filter 103, a low-frequency amplifier 104, a phase detector 105, a controller 106, a dc power supply 107, a combiner 108, and a low-frequency oscillator 109.
The combiner 108 superimposes a low-frequency signal from the low-frequency oscillator 109 on a high-frequency modulating signal or current. The laser diode 101 is driven by the modulating signal on which the low-frequency signal is superimposed. The photodiode 102 converts an output light from the laser diode 101 into an electrical signal. The low-pass filter 103 extracts a low-frequency component from the output signal of the photodiode 102. The low-frequency amplifier 104 amplifies the low-frequency signal. The phase detector 105 compares the phase of the amplified low-frequency component with the low-frequency signal from the low-frequency oscillator 109, and detects the slope of the laser output characteristic. The phase detector 105 generates a calibration signal for correcting the detected slope to a predetermined value, and outputs the calibration signal to the controller 106. The controller 106 controls the dc power supply 107 on the basis of the calibration signal. The combiner 108 changes the amplitude of the excitation current applied to the laser diode 101 on the basis of the dc voltage from the power supply 107. In this manner, the output amplitude of the laser diode 101 can be stabilized.
There is another optical modulation apparatus proposed by Japanese Laid-Open Patent Application No. 8-254672. This apparatus is illustrated in FIG. 2. The apparatus is made up of a laser diode (LD) 201, an optical modulator 202, a photocoupler 203, a light-receiving element 204, a band-pass filter (BPF) 205, a variable gain amplifier 206, a phase detector 207, a low-frequency oscillator 208, a dc amplifier 209, and a driver amplifier 210.
The optical modulator 202 modulates the intensity of the output light emitted from the laser diode 201 by a modulating signal on which a sine-wave signal from the low-frequency oscillator 208 is superimposed by the driver amplifier 210. The photocoupler 203 splits the modulated laser beam from the optical modulator 202. A split laser beam is applied to the light-receiving element 204, which converts it into an electrical signal. The band-pass filter 205 extracts a frequency component of the sine-wave signal from the electrical signal supplied thereto. The frequency component thus extracted is applied to the phase detector 207 via the variable gain amplifier 206. The phase detector 207 generates a bias current, which is controlled so as to stabilize the operating point on the basis of the phase difference between the frequency component from the variable gain amplifier 206 and the sine-wave signal from the low-frequency oscillator 208. The bias voltage is applied to the optical modulator 202 via the dc amplifier 209. In this manner, the intensity of the laser output is modulated by the optical modulator 202, so that the operating point of the laser output can be stabilized.
As described above, the conventional apparatuses described in the above-specified applications superimpose the low-frequency signal (pilot signal) on the optical main signal (high frequency signal of the order of a few GHz) in order to detect change of the quantum efficiency η, and thus cope with degradation due to temperature change and age deterioration.
However, the apparatuses cope with only variation of the slope of the quantum efficiency η caused by temperature change and age deterioration. More particularly, there are two factors that change the laser performance due to temperature change. These two factors are increase of the slope of the quantum efficiency η and increase of the threshold current for laser emission. The conventional apparatuses can cope with only increase of the slope of the quantum efficiency η. However, in actuality, the laser output is more greatly affected by increase of the threshold current for laser emission. The laser performance cannot be totally stabilized unless increase of the threshold current for laser emission due to temperature change and age deterioration is eliminated.
Japanese Laid-Open Patent Application No. 7-226714 discloses an optical transmitter capable of solving the above-mentioned problem. This optical transmitter is depicted in FIG. 3. As shown, the optical transmitter is made up of a laser diode (LD) 301, a photodiode (PD) 302, a current switch circuit 304, a D-type flip-flop (D-FF) 305, a capacitor C1, an amplifier 306, a variable resistor Rv1, an error amplifier circuit 307, a bias current circuit 308, a frequency dividing circuit 309, a waveform equalizing circuit 310, an attenuator 311, a current-voltage conversion amplifier circuit 312, a clip circuit 313, a low-frequency detector circuit 314, a capacitor C2, a variable resistor Rv2, and an error amplifier circuit 315.
The D-type flip-flop 305 latches input data (DATA) supplied from the outside of the optical transmitter in synchronism with a clock (CLOCK). The input data thus latched is applied to the current switch circuit 304. The current switch circuit 304 supplies current Ip to the laser diode 301. The photodiode 302 monitors the optical output level of the laser diode 301. The current-voltage conversion amplifier circuit 312 converts current flowing through the photodiode 302 into a voltage signal. The voltage signal is amplified by the amplifier circuit 306, and is then smoothed by the capacitor C1. The smoothed voltage is applied to the error amplifier circuit 307, which is supplied with a first reference voltage generated by the variable resistor Rv1. The error amplifier circuit 307 applies a signal based on the difference between the smoothed voltage and the first reference signal to the bias current circuit 308.
The frequency dividing circuit 309 divides the frequency of the clock supplied from the outside of the optical transmitter, and supplies the waveform equalizing circuit 310 with a resultant clock having a frequency lower than the external clock. The waveform equalizing circuit 310 converts the output waveform of the frequency dividing circuit 309, which is a rectangular waveform, into a sine wave. The attenuator 311 attenuates the sine wave, and applies the attenuated sine wave to the bias current circuit 308. The bias current circuit 308 generates a bias current for driving the laser diode 301 on the basis of the two inputs, and applies the bias current to the laser diode 301.
The voltage signal output by the current-voltage conversion amplifier circuit 312 is applied to the clip circuit 313, which clips the input voltage. The low-frequency detector circuit 314 extracts a low-frequency component from the output signal of the clip circuit 313. The low-frequency component thus extracted is smoothed by the capacitor C2, and is then applied to the error amplifier circuit 315, which is supplied with a second reference voltage generated by the variable resistor Rv2. The error amplifier circuit 315 compares the smoothed voltage with the second reference voltage, and amplifies the difference therebetween. The amplified difference voltage thus obtained is applied to the current switch circuit 304. In this manner, the current switch circuit 304 supplies the laser diode 301 with the current signal generated based on the signal from the D-type flip-flop 305 and the voltage signal from the error amplifier circuit 315.
As described above, the laser diode 301 in the optical transmitter shown in FIG. 3 is driven by not only the bias current generated based on the average of the optical output and the low-frequency component generated based on the clock, but also the current generated based on the average of the low-frequency component partially extracted from the optical output and the data signal. It is therefore possible to control not only the modulating current as well as the bias current and to cope with the problems resulting from both increase of the slope of the quantum efficiency η and increase of the threshold current for laser emission.
However, the optical transmitter shown in FIG. 3 has a disadvantage in that the circuit for superimposing the low-frequency signal having the fixed amplitude on the modulating current is very complex. This prevents downsizing and cost reduction.
In the case where the amplitude of the pilot signal (low-frequency signal) is fixed, the slope of the quantum efficiency η is inclined due to high temperature or age deterioration, the pilot signal superimposed on the optical signal gradually decreases irrespective of whether the amplitude of the driver output is increased. In case where the pilot signal decreases to a low level as compared to noise included in the optical signal, the feedback control using the monitor signal is no longer performed accurately, and the laser waveform is deformed.